Etching metal using sonication

ABSTRACT

A technique in accordance with the invention includes obtaining a semiconductor structure that has a metal disposed thereon. At least a portion of the metal is etched using an etching fluid while sonic energy is applied to the etching fluid.

BACKGROUND

[0001] The invention generally relates to etching metal usingsonication.

[0002] In a variety of different circumstances, it may be desirable toselectively etch metal in the formation of a semiconductor device. Forexample, the etching of metal may be related to the formation of a metalsilicide layer (a nickel silicide layer, for example), a layer used toreduce metal-to-semiconductor contact resistances in a semiconductordevice.

[0003] To form a metal silicide layer, a metal layer (nickel, forexample) typically is deposited on a semiconductor structure. In thismanner, the deposited metal reacts with exposed silicon of the structureto form the metal silicide layer. Not all of the deposited metal layertypically reacts. In this manner, the regions in which the metal layerdoes not react form excess or un-reacted metal regions that typicallyare removed by wet etching. As a more specific example, FIG. 1 depicts asemiconductor structure 9 that represents a particular stage in aprocess to form a complimentary metal oxide semiconductor (CMOS)transistor. For this example it is assumed that the CMOS transistor isformed on a silicon substrate 12. As shown in FIG. 1, the polysiliconlayer 18 resides on top of a gate oxide layer 16, and verticallyextending nitride spacers 20 may be located on either side of thepolysilicon layer 18.

[0004] For purposes of creating a nickel silicide layer, a nickel layer22 may be blanket deposited over existing layers of the structure 9. Asdepicted in FIG. 1, the deposited nickel layer 22 extends over portionsof the silicon substrate 12 as well as extends over a polysilicon layer18. The regions in which the nickel layer 22 contacts the siliconsubstrate 12 form parts of the source and drain of the transistor, andthe region in which the nickel layer 22 contacts the polysilicon layerforms part of the gate of the transistor in this example.

[0005] Thus, the deposited nickel layer 22 contacts the polysiliconlayer 18 and the silicon substrate 12, and in these contacted regions,the nickel layer 22 reacts with the polysilicon layer 18 and the siliconsubstrate 12 to form the nickel silicide layer that extends into regions26 of a resulting structure 10 that is depicted in FIG. 2. As a morespecific example, a particular nickel silicide region 26 a may beassociated with a drain of the transistor, another nickel silicideregion 26 b may be associated with a source of the transistor, andanother nickel silicide region 26 c may be associated with a gate of thetransistor.

[0006] The deposited nickel does not react everywhere, leaving regions24 of excess or unreacted nickel. To remove these regions 24, selectivewet etching is used to target the nickel but not other substances (suchas nickel silicide, for example) to remove the nickel to form astructure 11 that is depicted in FIG. 3. Thus, after the selective wetetching, the unreacted nickel portions 24 (see FIG. 2) are removed,leaving only the regions 26 of nickel silicide film, as depicted in FIG.3.

[0007] The wet etching typically involves submersing a wafer thatcontains the structure 10 into a nickel selective etchant, or etchingfluid, that typically includes both an acid, such as sulfuric acid, andan oxidant, such as hydrogen peroxide or nitric acid. At roomtemperature, the use of sulfuric acid by itself to etch the nickel isnot sufficient due to the potential energy barrier that prevents theoxidation of the nickel in accordance with the Pourbaix chart fornickel. Therefore, an oxidant typically is introduced into the etchingfluid to supply the needed energy to oxidize the nickel into an aqueousderivative and thus, dissolve the nickel.

[0008] For certain semiconductor devices, an oxidant in the etchingfluid may undesirably oxidize and thus, etch substances that are notmeant to be etched. For example, elemental germanium substrates,germanium-doped silicon substrates and germanide films are examples ofgermanium-based substances that typically are highly susceptible tooxidants that are used in the etching of nickel. The etch rates forthese germanium substances may be the same or even higher than the etchrate for nickel in the presence of such an oxidant. Therefore, whengermanium-based substances are present, the use of conventional etchingfluid to etch nickel may undesirably dissolve significant portions ofthese germanium-based substances.

[0009] Thus, there is a continuing need for a better way to selectivelyetch metal that is disposed on a semiconductor structure that containscertain semiconductor substrates, films and/or layers.

BRIEF DESCRIPTION OF THE DRAWING

[0010]FIGS. 1, 2 and 3 are cross-sectional views of semiconductorstructures depicting different stages in the formation of asemiconductor device according to the prior art.

[0011]FIG. 4 is a flow diagram depicting a technique to form asemiconductor structure according to an embodiment of the invention.

[0012]FIGS. 5, 6, 7 and 8 are cross-sectional views of semiconductorstructures in accordance with embodiments of the invention depictingdifferent stages in the formation of a semiconductor device.

DETAILED DESCRIPTION

[0013] Germanium-based substances (herein called “germaniumsubstances”), such as germanide films, germanium-doped regions andelemental germanium substrates, may be highly susceptible to theetchant, or etching fluid, that is conventionally used to etch nickel.In this manner, a typical etching fluid for nickel contains an acid,such as sulfuric acid, and an oxidant, such as hydrogen peroxide ornitric acid, which are highly oxidizing in nature. Although this etchingfluid may be used in a standard silicon-based process, the etching fluidundesirably etches germanium substances because germanium is highlysoluble in a low pH, aqueous solution that contains an oxidant (hydrogenperoxide or nitric acid, as examples).

[0014] Thus, if such an oxidant-containing etching fluid is used to etchnickel that is disposed on a semiconductor structure that includesgermanium substances, the germanium substances may be undesirablydissolved. However, an etching fluid that lacks an oxidant is not byitself sufficient to etch nickel due to the potential energy barrierthat exists for dissolving nickel (i.e., oxidizing nickel to someaqueous nickel derivative) in a low pH solution. To address thisproblem, an embodiment of a technique in accordance with the inventionovercomes the potential energy barrier by applying sonic energy to anoxidant-free etching fluid. Thus, with the application of sonic energyto oxidant-free etching fluid during etching of nickel, the nickel maybe selectively etched while germanium substance(s) of the semiconductorstructure remain intact.

[0015] Therefore, referring to FIG. 4, an embodiment 100 of a techniquein accordance with the invention includes depositing (block 102) a metallayer on a semiconductor structure. This metal layer may be, forexample, a nickel layer, that reacts with germanium regions of thestructure to form a nickel germanide film, or layer. This nickelgermanide layer, in turn, may be located between germanium substances ofthe structure and source and drain metal contacts for purposes ofreducing contact resistances between the germanium substances and thesecontacts. The nickel layer may also be deposited for purposes of forminga nickel silicide layer between a polysilicon layer and a gate metalcontact for purposes of reducing a contact resistance between thepolysilicon layer and the gate metal contact.

[0016] After the metal layer to form the germanide layer (and possibly asilicide layer) is deposited in accordance with the technique 100, theresulting metal germanide and silicide regions are annealed, as depictedin block 104. Subsequently, in accordance with the technique 100, thestructure is selectively wet etched with an oxidant-free etchant, oretching fluid, to remove the excess or unreacted metal regions(unreacted or excess nickel regions, for example) while sonic energy isapplied to the etching fluid to supply sufficient energy to facilitateoxidation of the metal being etched, as depicted in block 106. Theetching fluid may include sulfuric acid, for example. Due to the lack ofan oxidant in the etching fluid, undesirable etching of germaniumsubstances of the structure does not occur.

[0017] As a more specific example, FIGS. 5, 6, 7 and 8 depictsemiconductor structures that represent different stages in theformation of a CMOS transistor, in accordance with some embodiments ofthe invention. More specifically, FIG. 5 depicts a semiconductorstructure 118, in accordance with an embodiment of the invention, thatis formed on a germanium substrate 122. The substrate 122 may be anelemental germanium substrate. Alternatively, the substrate 122 may be asilicon substrate that is doped with germanium in the source and drainregions of the transistor.

[0018] Regardless of how the germanium is introduced, the germaniumsubstrate 122 includes a first region 125 that may be associated with asource of the transistor and another region 127 that may be associatedwith a drain of the transistor. The germanium substrate 122 is isolatedon either side by insulating oxide regions 124.

[0019] The germanium substrate 122 may also include a region 129 that isassociated with a gate of the transistor. A gate oxide layer 134 isdeposited directly on the germanium substrate 122 on the gate region129, and a polysilicon layer 128 is formed on top of the gate oxidelayer 134. Nitride spacers 126 may extend upwardly on either side of thepolysilicon layer 128. Alternatively, the polysilicon layer 128 may bereplaced by a germanium-based, germanium-silicon-based or metal-basedlayer, as just a few examples.

[0020] As depicted in FIG. 5, a layer 130 of nickel is blanket depositedon the structure 118 and covers the otherwise exposed germaniumsubstrate 122 and the otherwise exposed polysilicon layer 128. Reactionsoccur with the nickel to form a structure 119 that is depicted in FIG.6.

[0021] Referring to FIG. 6, in this manner, the nickel reacts with theexposed polysilicon 128 and the exposed germanium substrate 122 to formnickel germanide regions 142 over the exposed germanium substrate 122and a nickel silicide region 140 over the exposed polysilicon layer 128.Thus, the nickel silicide region 140 is formed from the reaction ofsilicon (in the polysilicon layer 128) with the nickel, and the nickelgermanide regions 142 are formed by the reaction of germanium (in thegermanium substrate 122) with the nickel. Therefore, the reactions withthe deposited nickel layer 130 form one nickel germanide region 142 athat is associated with the drain of the transistor, another nickelgermanide region 142 b that is associated with source of the transistorand the nickel silicide region 140 that is associated with the gate ofthe transistor.

[0022] As illustrated in FIG. 6, not all of the nickel reacts, therebyleaving unreacted or excess nickel regions, such as the depicted regions146. A next step in the process to form the transistor may be theannealing of the nickel silicide region 140 and the nickel germanideregions 142 a and 142 b. After the annealing, the structure 119 isselectively wet etched in an oxidant-free etchant, or etching fluid,such as sulfuric acid, for example. During this etching, sonic energy(in lieu of the inclusion of an oxidant in the etching fluid) is appliedto the etching fluid for purposes of overcoming the high energy barrierthat is associated with the dissolution of nickel in solutions of lowpH.

[0023] As a more specific example, in some embodiments of the invention,ultrasonic sonic energy in the frequency range between approximately 10kilohertz (kHz) and 100 kHz may be applied to the etching fluid duringthe etching of the unreacted nickel. Alternatively, in some embodimentsof the invention, megasonic energy in the range of approximately 500 to1000 kHz may be applied to the etching fluid during the etching of thenickel. The sonic energy may be applied via transducers that are locatedin, on or near an immersion tank in which the structure 119 is immersedand the wet etching is performed.

[0024] The result of the etching is a structure 120 that is depicted inFIG. 7. In this manner, the etching removes the unreacted nickel regions146 (FIG. 6) to leave the nickel silicide region 140 located above thepolisilicon layer 128 and the nickel germanide regions 142 a and 142 bof the drain and source regions, respectively.

[0025] Many other steps may be performed in the process to form thetransistor from the structure 120. As an example of one out of possiblymany more steps that may be performed, in some embodiments of theinvention, an oxide layer 160 may be subsequently deposited on thestructure 120 to form a structure 121 that is depicted in FIG. 8. Theoxide layer 160 is polished back and then selective etching is performedto create contact holes so that a metal layer may be deposited to formcorresponding transistor contacts 162 with the germanide and silicidefilms. For example, as depicted in FIG. 8, the structure 121 may includea source metal contact 162 a that extends through a contact hole in theoxide layer 160 to the nickel germanide region 142 b, and the structure121 may include a drain metal contact 162 b that extends through anothercontact hole in the oxide layer 160 to contact the nickel germanideregion 142 a. A gate metal contact may be also formed to the nickelsilicide region 140, although such a contact is not depicted in thecross-section illustrated in FIG. 8.

[0026] Thus, due to the intervening nickel germanide and silicidelayers, contact resistances are decreased between upper metal layers andthe germanium substrate 122 and polysilicon layer 128. As an example,tungsten may be used to form the metal contacts 162. Other metals may beused.

[0027] In the context of this application, although the precedingdescription may have used such terms as “over” and “on” to describe therelative positions or locations of certain substances, materials orlayers these terms do not necessarily mean that the substances,materials or layers contact each other, unless otherwise stated.

[0028] While the present invention has been described with respect to alimited number of embodiments, those skilled in the art, having thebenefit of this disclosure, will appreciate numerous modifications andvariations therefrom. It is intended that the appended claims cover allsuch modifications and variations as fall within the true spirit andscope of this present invention.

What is claimed is:
 1. A method comprising: obtaining a semiconductorstructure having a metal disposed thereon; and etching at least aportion of the metal using an etching fluid while applying sonic energyto the etching fluid.
 2. The method of claim 1, further comprising:depositing a metal layer on the structure, the deposited metal layerforming reacted and unreacted metal regions, wherein the etchingcomprises etching at least a portion of the unreacted metal regions. 3.The method of claim 1, wherein the obtaining comprises obtaining asemiconductor structure having a germanium substrate.
 4. The method ofclaim 1, wherein the obtaining comprises obtaining a semiconductorstructure having a region containing germanium.
 5. The method of claim4, wherein the obtaining comprises obtaining a semiconductor structurehaving nickel disposed thereon, and the etching comprises etching atleast a portion of the nickel while applying sonic energy to the etchingfluid.
 6. The method of claim 1, wherein the obtaining comprisesobtaining a semiconductor structure having nickel disposed thereon, andthe etching comprises etching at least a portion of the nickel whileapplying sonic energy to the etching fluid.
 7. The method of claim 1,wherein the obtaining comprises obtaining a semiconductor structurehaving a germanium region and nickel disposed over the substrate.
 8. Themethod of claim 1, wherein the applying the sonic energy comprisesapplying ultrasonic energy.
 9. The method of claim 1, wherein theapplying sonic energy comprises applying megasonic energy.
 10. Themethod of claim 1, wherein the etching comprises etching without usingan oxidant in the etching fluid.
 11. A method comprising: obtaining asemiconductor structure having nickel disposed thereon and a regioncontaining germanium; and etching at least some of the nickel using anetching fluid while applying sonic energy to the etching fluid.
 12. Themethod of claim 11, further comprising: depositing the nickel on thesemiconductor structure to form nickel germanide in at least one regionand unreacted nickel in another region; and etching to remove at leastsome of the unreacted nickel.
 13. The method of claim 11, wherein theobtaining comprises obtaining a semiconductor structure having agermanium substrate.
 14. The method of claim 1, wherein the obtainingcomprises obtaining a semiconductor structure having a silicon substratehaving at least one germanium region.
 15. The method of claim 11,wherein the etching comprises etching without using an oxidant in theetching fluid.
 16. The method of claim 11, wherein the applying thesonic energy comprises applying ultrasonic energy.
 17. The method ofclaim 11, wherein the applying sonic energy comprises applying megasonicenergy.
 18. A method comprising: obtaining a semiconductor structurehaving a germanium region and a metal disposed on the semiconductorstructure; and etching at least a portion of the metal while applyingsonic energy to an etching fluid.
 19. The method of claim 18, furthercomprising: depositing a metal layer on the semiconductor structure toform a metal germanide in a first region and unreacted metal in a secondregion, wherein the etching comprises etching at least a portion of thesecond region.
 20. The method of claim 18, wherein the obtainingcomprises obtaining a semiconductor structure having a germaniumsubstrate.
 21. The method of claim 18, wherein the obtaining comprisesobtaining a semiconductor structure having a silicon substrate having agermanium region.
 22. The method of claim 18, wherein the applying thesonic energy comprises applying ultrasonic energy.
 23. The method ofclaim 18, wherein the applying the sonic energy comprises applyingmegasonic energy.
 24. A method comprising: obtaining a semiconductorstructure having a region capable of being dissolved by a first etchingfluid that includes an oxidant; and etching at least a portion of alayer deposited on the substrate using a second etching fluid that doesnot include the oxidant while applying sonic energy to the secondetching fluid.
 25. The method of claim 24, wherein the obtainingcomprises obtaining a substrate having a germanium region capable ofbeing dissolved by the first etching fluid.
 26. The method of claim 24,wherein the application of the sonic energy provides energy to dissolvesaid at least a portion of the layer.
 27. The method of claim 24,wherein the applying the sonic energy comprises applying ultrasonicenergy.
 28. The method of claim 24, wherein the applying the sonicenergy comprises applying megasonic energy.
 29. The method of claim 24,wherein the etching at least a portion of a layer comprises etching atleast a portion of a metal layer.
 30. The method of claim 24, whereinthe etching at least a portion of a layer comprises etching at least aportion of a nickel layer.
 31. A method comprising: etching at leastsome of a metal disposed on a semiconductor structure using anoxidant-free etching fluid; and applying sonic energy to the etchingfluid while etching.
 32. The method of claim 31, wherein the etchingcomprises etching nickel.
 33. The method of claim 31, wherein theetching comprises etching metal disposed on a semiconductor structurecomprising a germanium region.
 34. The method of claim 31, wherein theapplying the sonic energy comprises applying ultrasonic energy.
 35. Themethod of claim 31, wherein the applying the sonic energy comprisesapplying megasonic energy.
 36. A semiconductor structure comprising: asubstrate containing a germanium region; a metal contact; and agermanide layer located between the germanium region and the metalcontact.
 37. The semiconductor structure of claim 36, wherein thegermanide layer contacts the metal contact and the germanium region. 38.The semiconductor structure of claim 36, wherein the germanide layercomprises a nickel germanide layer.
 39. The semiconductor structure ofclaim 36, wherein the germanide layer comprises a silicon germanidelayer.
 40. The semiconductor structure of claim 36, wherein the metalcontact is associated with one of a source and a drain of a transistor.